Bronze dolphin shaped Spatha or shorter semi-spathascabbard slide.
This piece would have been attached to the scabbard or sheath of the 3rd
or 4th Century Spatha or Semi-spatha. (see reference pictures below on how
it was mounted)

It would have provided the loop required to suspend the sword from the
shoulder using the baldric suspension method. (2)

An identical example is listed here (3) discovered in South Shields,
England where there was a long Roman occupation of the 3rd Century.

The design is in the shape of a dolphin with jagged fins along the top and
both sides as well as the tail. "The dolphin is an ancient symbol in Greek
and Roman art associated with Oceanus and his Roman counterpart Neptune.
Dolphins can be seen on many artifacts from ceramic oil lamps through to
mosaic floors as at Fishbourne." (1)

A
stable patina covers the piece originally most likely some type of
copper/bronze alloy. A small section is missing from the "tail" and
was clearly broken during ancient times. A unique and rare nearly complete example.